SORT1 blocking antibodies for Alzheimer's disease and Frontotemporal dementia Alector's objective is to develop therapeutic antibodies against the receptor Sortilin1 (SORT1) for the treatment of the devastating neurological disorders Frontotemporal Dementia (FTD) and Alzheimer's Disease (AD). FTD is a progressive disease that causes incapacitating changes in behavior, language, movement and cognition, and is the most common of the pre-senile dementias, affecting ~50,000-60,000 people in the US. AD is the most common cause of dementia, which shares many pathological and phenotypic features with FTD. As AD affects an estimated 5M Americans, with the numbers increasing steadily, the disease represents a major public health issue. There is no effective treatment for FTD, which is typically diagnosed 3-4 years after symptom onset, with a median survival of 6-11 years. Furthermore, there are no available treatments that halt progression of AD. Ultimately, patients with FTD or AD will require round-the-clock medical care. Progranulin (PGRN) is a neurotrophin that has been identified as a risk factor for neurodegenerative diseases. PGRN haploinsufficiency is causal for 10% of all FTD cases, while other alleles are associated with the development of late onset AD. SORT1 is a transmembrane receptor that controls the extracellular level of PGRN by binding it at the cell surface and rapidly internalizing it for lysosomal degradation, while it is dispensable for PGRN signaling. We propose to generate a therapeutic that can elevate extracellular levels of PGRN by developing antibodies that bind SORT1, block the interaction with PGRN, and thus functionally elevate PGRN levels in vitro and in vivo. Effective lead antibodies will be optimized for affinity and other characteristics and advanced for confirmatory and preclinical testing as development candidates. Both the dataset and SORT1 antibodies produced in this project would allow us to secure the additional funds necessary for advancing the candidate to preclinical testing, to IND submission, and to the clinic. Trials with a SORT1 antibody in patients with PGRN mutations would conclusively determine whether therapeutic PGRN elevation could provide meaningful clinical benefit in FTD and AD patients as hypothesized.